Image sensors are used to produce an image representing an object. The image sensors include rows and columns of pixels. The pixels generate small photo signals proportional to light reflected from an object to be imaged. The photo signal is read and processed by signal processing circuitry to create an image representing the object.
Pixels belonging to the same column (also referred to as bitline) are usually connected at a common output node from where the signal is read out. Each pixel in a same bitline is individually controlled to read out at the common output node. At the output node, a column readout circuit is provided to read out and amplify the photo signal. A column readout circuit is also referred to as a line buffer.
Typically, a pixel includes a driving device that receives an electronic signal indicative of an intensity of light detected by the image sensor and drives a current proportional to the intensity (the photo signal), to a bitline to which the pixel cell is coupled. Following signal integration, pixels of a selected row are accessed by asserting a row select signal to each pixel of the selected row.
Additionally, the column readout circuit, in some image sensors, is used to remove thermal noise, fixed pattern noise, and other types of noise. This is done by having the column readout circuit sample the output of the pixel during a reset period. The column readout circuit then subtracts the reset signal from the photo signal. This type of readout circuit is sometimes referred to as a correlated double sampling circuit. In some prior art image sensors, a second stage column readout circuit is used to further amplify the photo signal and to eliminate noise caused by the first stage column readout circuit. An example of a correlated double sampling column readout circuit is seen in U.S. Pat. No. 6,222,175. Another example is shown in our co-pending U.S. patent application Ser. No. 10/109,142 filed Mar. 27, 2002 entitled COLUMN READOUT CIRCUIT WITH INCREASED SIGNAL RANGE FOR CMOS IMAGE SENSOR.
In the prior art, one column readout circuit is associated with each column of the pixel array. For a pixel array with thousands of columns, this requires thousands of column readout circuits. As the drive for smaller and smaller integrated circuits continues, the large number of column readout circuits (which are relatively large), may require an inordinate amount of silicon real estate.
In certain applications, such as a linear sensor, the column readout circuits may make up the bulk of the entire circuit. Further, the large number of column readout circuits may require a large amount of current. A linear sensor is generally characterized as having a pixel array that has much more columns than rows. For example, a linear sensor may have less than five rows, yet thousands of columns. One application for a linear sensor is for an optical scanner or reader.